356 ELEMENTS OF ELECTRICAL ENGINEERING. 



circuit, and (JZ) is the proportionality factor. The quantity L 

 is called the inductance of the given circuit. 



Moment of inertia of a wheel. Analog of inductance. The 

 kinetic energy of a rotating wheel resides in the various moving 

 particles of the wheel. If the angular velocity o> of the wheel 

 is doubled, the linear velocity of every particle of the wheel is 

 doubled, so that the kinetic energy of every particle is quadrupled. 

 Therefore, the total kinetic energy of a wheel is quadrupled when 

 the angular velocity is doubled, so that the kinetic energy of a 

 given wheel is proportional to the square of the angular velocity, 

 that is 



W = JAV 



in which W is the kinetic energy of a rotating wheel, o> is the 

 angular velocity of the wheel, and (JA") is the proportionality 

 factor. The quantity K is called the moment of inertia of the 

 wheel. 



Units of inductance. If W in equation (i) is expressed in 

 joules and i in amperes, then L is expressed in terms of a 

 unit of inductance which is called a henry. That is to say, a 

 circuit has an inductance of one henry when the kinetic energy 

 of one ampere of current in the circuit is one half of a joule. 



If W in equation (i) is expressed in ergs, and i in abamperes, 

 then L is expressed in terms of the c.g.s. unit of inductance. 

 This c.g.s. unit of inductance is usually called the centimeter for 

 the reason that the square of a current must be multiplied by a 

 length to give energy or work ; that is, inductance is expressed 

 as a length and the unit of inductance is of course the unit of 

 length. 



Inductance of a coil. Strictly, one cannot speak of the induc- 

 tance of anything but an entire circuit, inasmuch as every portion 

 of a circuit contributes its share to the magnetic field at each and 

 every point ; it is, however, allowable to speak of the inductance 

 of a coil when the terminals of the coil are near together, and 

 when the remainder of the electric circuit does not produce any 

 perceptible magnetic field in the region occupied by the coil. 



